1. Field of the Invention
The present invention relates to an electron gun for a color cathode ray tube, and more particularly, to an electrode system for controlling an electrostatic field in an electron gun for a color cathode ray tube, which can improve astigmatisms and OCV(Outer Beam Convergence Variance), particularly on periphery of the screen, that occur when electron beams are deflected, to improve the resolution of the color cathode ray tube.
2. Discussion of the Related Art
The electron gun in a color cathode ray tube is an electron beam emitting device which forms a pixel by focusing three electron beams emitted from respective cathodes onto red, green and blue fluorescent surfaces at a front part of the cathode ray tube such that each of the surfaces reacts with a respective electron beams, thereby forming an image on the screen through combination of the pixels.
FIG. 1 illustrates an outline of the color cathode ray tube provided with a conventional in-line type electron gun.
Referring to FIG. 1, the color cathode ray tube has a panel 1 of glass forming a front surface thereof and a funnel 2 of which the front portion is fusion welded to a rear portion of the panel 1. The funnel, converges to form a neck portion 2a at the rear end of the tube in which an electron gun 3 is sealed. There is a fluorescent surface 5 on the inside of the panel 1 having red, green, blue fluorescent materials coated thereon for illumination by electron beams 4 emitted from the electron gun. There is also a shadow mask 6 that is perforated with electron beam pass-through holes 61 for selective pass through of the three electron beams 4 and is spaced a certain distance apart from the panel 1. There are deflection yokes 7 on an outer circumference of the neck portion 2a for deflecting the electron beams 4 to the panel 1, i.e., to regions of the screen.
FIG. 2 illustrates the conventional in-line type electron beam shown in FIG. 1 with a partial cut-away view.
Referring to FIG. 2, the conventional electron gun includes three cathode ray electrodes 8 each having a heater (not shown), a controlling electrode 9 which is a first grid electrode for controlling the electron beams, an accelerating electrode 10 which is a second grid electrode for accelerating the electron beams, pre-focus electrodes 11 and 12 which are third and fourth grid electrodes for pre-focusing the electron beams, a focusing electrode and anode 13 and 14 which are fifth and sixth grid electrodes for finally focusing and accelerating the electron beams, and a shield cup 16 disposed at one end of the anode 14 in the direction of the screen for shielding leakage magnetic fields from the deflection, and the foregoing electrodes are fixed by one pair of bead glass with predetermined distances spaces between them. The focusing electrode 13 has a first focusing electrode 131 to which a static voltage is applied and a second focusing electrode to which a dynamic voltage is applied.
In the operation of the electron gun, when a predetermined voltage is applied to each of the electrodes and currents are applied to the cathode electrodes 8, heaters in the cathode electrodes 8 are heated to emit thermal electron beams 4, which are accelerated toward the screen by a voltage difference between the accelerating electrode 10 and the controlling electrode 9. Then, the electron beams 4 are pre-focused by the pre-focusing electrodes 11 and 12 and finally focused and accelerated by a main electrostatic focusing lens formed by a voltage difference between the second focusing electrode 132 and the anode 14. Thereafter, the electron beams 4 are deflected by the deflection yokes 7, pass through the electron beam pass-through holes 61 in the shadow mask 6, and collide onto the fluorescent surface to form a pixel. The larger the size of the main electrostatic focusing lens, the more exact the focusing of the electron beams, resulting in a sharper image on the screen. However, the small diameter of about 5.5.about.5.9 mm of the main focusing electrostatic lens causes a spherical aberration, which causes hazing of the electron beams that degrades the resolution of the color cathode ray tube. The spherical aberration is proportional to an inverted third power of the diameter of the main electrostatic focusing lens, and the diameter of the main electrostatic focusing lens is substantially proportional to diameters of the electron beam pass-through holes in the second focusing electrode 132 and the anode 14. Therefore, in general, to lower the spherical aberration, it has been suggested that the diameters of the electron beam pass-through holes in the second focusing electrode 132 and the anode 14 should made greater, resulting in a larger main electrostatic focusing lens.
FIG. 3 illustrates a perspective view of an example of a conventional second focusing electrode 132 and the anode 14 in a partial cut away view FIG. 4 illustrates a frontal section of the system shown in FIG. 3, together with the neck portion for reference.
Referring to FIGS. 3 and 4, the diameter of each of the three electron beam pass-through holes 132c and 132s, in the second focusing electrode 132 and 14c and 14s in the anode 14 respectively formed on a plane perpendicular to a center axis of the neck portion 2a is limited to less than 1/3 of the inside diameter of the neck portion 2a, because the second focusing electrode 132 and the anode 14 should be disposed in the neck portion 2a. Accordingly, in the aforementioned electron gun for a color cathode ray tube, in order increase the diameter, D, of the electron beam pass-through holes 132c, 132s, 14c and 14s that form the main electrostatic focusing lens, the inside diameter L of the neck portion 2a should be made greater, the gap g between the outside circumferences of the second focusing electrode 132 and the anode 14 and the neck portion 2a, and the widths I, and I.sub.2 of the bridges of the electron beam pass-through holes 132c and 132s, 14c and 14s should be minimized. The distances between the electron beam pass-through holes 132c, 132s, 14c and 14s, i.e., the beam separation S should be made greater. However, there are limitations placed on the reduction of the gap, because an electrical insulation should be maintained between the second focusing electrode 132, the anode 14 and the neck portion 2a. There are limitations on the reduction of the widths 1.sub.1 and 1.sub.2 of the bridges because of strength of the bridges. Also, cases in which the inside diameter L of the neck portion 2a is made greater and the beam separations S are made greater, causes higher deflection power consumption for the deflection yokes and degraded resolution due to weakened convergence of the electron beams, due to the greater beam separation S. Therefore, a design that can provide the largest electron beam pass-through holes D, while the inside diameter L of the neck portion 2a is maintained, is required.
FIG. 5 illustrates a perspective view of another example of a conventional second focusing electrode 132 and anode 14 having electrostatic field controlling electrodes provided therein, with a partial cut away view, and FIG. 6 illustrates a section of the conventional second focusing electrode 132 and anode 14 shown in FIG. 5, wherein the same reference numbers are used for identical parts explained before.
Referring to FIGS. 5 and 6, electrode barrels 132d and 14d and electrostatic field controlling electrodes 17 and 18 are disposed in respective electrode barrels and adapted to receive the same voltage as the respective electrode barrel. Outer ends of the electrode barrels 132d and 14d are opened such that the three electron beams may pass in common, and inner ends thereof, disposed oppositely, are also opened in the same manner. The inner ends each have a rim portion 132e and 14e formed thereon along an inside circumference, with an inside wall of a predetermined length extended inwardly into the second focusing electrode 132 and the anode 14. Each of the electrostatic field controlling electrodes 17 and 18, disposed at a position away from the rim portion 132d and 14d by a predetermined distance c and a and arranged vertically with respect to the direction of travel of the electron beams, includes a flat portion 17b and 18b having a center electron beam pass-through hole 17a and 18a, and blades 17c and 18c bent at a right angle to the flat portion 17b and 18b at both ends of the flat portion 17b and 18b.
Accordingly, the center electron beam entering into the second focusing electrode passes through the center electron beam pass-through hole 17a, and the outer electron beams pass through the spaces between the inside of the electrode barrel 132d and the blade 17c. The electron beams then pass through the anode in the same manner as the second focusing electrode. In this case, as the openings defined by the rim portions 132f and 14f of the second focusing electrode 132 and anode 14 are large in diameter, the diameter of the main focusing electrostatic lens can be made large, but with the horizontal diameter being much larger than the vertical diameter. Because of this, the horizontal focusing power is significantly weakened compared to the vertical focusing power, which changes the focus distance and causes an astigmatism. However, the electrostatic field controlling electrodes 17 and 18 project an electrostatic field into the openings, which prevents the occurrence of the astigmatism to some extent. The additional fields formed by the blades 17c and 18c, which have certain widths at both sides of the center electron beam pass-through holes 17a and 18a affect, the horizontal focusing power of the main focusing electrostatic lens. As the positions of the electrostatic field controlling electrodes 17 and 18 are deeper in the second focusing electrode 132 and the anode 14, i.e., the farther from the rim portions 132e and 14e, the electric field between the two electrostatic field controlling electrodes 17 and 18 becomes weaker with formation a greater slope of equipotential lines, and the diameter of the main focusing electrostatic lens can be increased.
However, the deeper positioning of the electrostatic field controlling electrodes for obtaining a larger diameter main focusing electrostatic lens causes the following problems.
First, the deeper positioning of the electrostatic field controlling electrode in the second focusing electrode results in a negative tendency of the astigmatism, i.e., underfocusing of the electron beams in the horizontal direction and overfocusing in the vertical direction. Causing a vertical dispersion of the image and reducing the OCV, which represents a convergence of outer beams.
Second, the deeper positioning of the electrostatic field controlling electrode in the anode results in a positive tendency of the astigmatism, i.e., overfocusing of the electron beams in the horizontal direction and underfocusing in the vertical direction. Causing a horizontal dispersion of the image and increasing the OCV, which represents a convergence of outer beams.
Even though the aforementioned problems can be solved to some extent by positioning the electrostatic field controlling electrodes appropriately, there is a limitation on the extent of the improvements in astigmatism and OCV that can be realized solely by adjustment of the positions of the electrostatic field controlling electrodes.